Arising from: Hanauer SB (2007) Nat Clin Pract Gastroenterol Hepatol 4: 469

Because of our respective interests in the MMR vaccine (BJ Ward) and mycobacteria (MA Behr), we would like to respond to the editorial by Stephen B Hanauer, entitled “More likely than not”, which discussed the assessment of causality in courts of law.1 We wholeheartedly agree that attempts to assign medical causality in the courts are problematic; however, we take issue with lumping the ongoing debate about the possible association between Mycobacterium avium paratuberculosis and Crohn's disease with the discredited MMR-autism hypothesis.

No matter what the National Vaccine Injury Compensation hearing concludes, the scientific community's verdict is already in for MMR-autism. The data published in support of the MMR-autism hypothesis are remarkably poor and there are abundant, high-quality studies refuting the alleged association between the MMR vaccine and autism.2 This is not the situation with M. avium paratuberculosis-Crohn's disease, where the jury is still out on whether or not there is a causal association between M. avium paratuberculosis and Crohn's disease. A systematic review and meta-analysis published in 2007 found an odds ratio of 7.0 for the detection of M. avium paratuberculosis DNA in tissue from Crohn's disease patients.3 The linkage and simultaneous dismissal of the Mycobacterium avium paratuberculosis-Crohn's disease hypothesis with the MMR-autism hypothesis obscures profound differences between these hypotheses and available data.

Scientific debate is best carried out in the pages of high-quality journals such as Nature Clinical Practice Gastroenterology & Hepatology. Where a reasonable degree of scientific equipoise exists, we hope that this journal (and others) will welcome contributions that encourage open discussion, further investigation and eventual resolution.